1. Field
This invention relates to capacitive sensing and capacitance measuring, specifically to capacitive key touch detection.
Capacitive sensing detects the presence or absence of a conductive object, such as a human finger.
2. Prior Art
Capacitive touch sensing technology has a wide range of applications in electronic equipment. A touch sensing controller IC (integrated circuit) is the key component in touch sensing enabled systems, such as equipment with touch keys or a touch screen. As the touch sensing technology is needed by many low-end applications, the cost of the sensing controller becomes more sensitive. In another aspect, as the controller ICs are more relatively affordable, more new applications will be seen for touch sensing technology. Also, the price is always a competitive factor in market. As always, a simpler design and less pin count of IC components can bring the cost down for development and fabrication. There are some capacitive touch sensing technologies that are popularly used in the real product. Notable among those are:
1. Relaxation oscillator method. In this method, the capacitive sensing element is part of a relaxation oscillator. The capacitance and the change can be calculated by measuring the oscillation frequency.
2. ADC based methods. The voltage amplitude of the capacitor at the certain time during charge or discharge is sampled by ADC. The amplitude and it differences can be used to calculate the capacitance and its change.
3. Switched-capacitor or charge transfer. The capacitors with sequential-controlled switches are used. This kind of approaches is described in the documents such as U.S. Pat. Nos. 6,466,036 B1, and 7,015,705.
The existing capacitive touch sensing methods are seen to have disadvantages and problems, which are listed below, especially in touch key applications and when compared to the mechanical key detection, which uses digital input ports to detect open/short resistive contact for key press and timers to handle de-bouncing.
a) analogue design technology has to be used for designing comparator, ADC (analogue to digital converter), an analog multiplexer, etc. The design of a mixed-signal IC, which has both digital and analog circuitry, requires a high level of expertise and careful use of computer aided design (CAD) tools. Automated testing of the finished chips can also be challenging.
b) each key needs multiple pins for the sensing controller.
c) control and measurement ports or pins need to be switched to different modes, which adds the complexity of the circuit design or system design. Also the fast switching control from switched-capacitor may bring noise and EMI issues potentially.
d) when the port to connect keys are not used, it may not be able to use as other purposes.
e) each method is only suitable to be used for a certain number range of keys because of per-key cost. A component vender has to have different production lines and use different technologies to cover the applications with different number of keys.